Increasing numbers of states and professional organizations are recommending and even mandating systematic screening for developmental and behavioral problems, including autism, among young children in pediatric settings. Unfortunately there is a weak evidence base from which pediatricians are forced to make decisions to inform such screening. A comparative effectiveness trial of screening instruments that can detect developmental-behavioral problems in children less than 5 years of age is urgently needed. To address the considerable gaps in the research literature on screening, we aim to conduct a large, head-to- head comparative effectiveness trial of 3 sets of developmental-behavioral screening instruments for children under 5 years of age (the Ages and Stages, the Parent's Evaluation of Developmental Status, and our own instrument, the Survey of Wellbeing on Young Children). All screening instruments will be administered twice, to provide evidence for the benefits of follow up with repeated screening. A comprehensive clinical evaluation will allow us to analyze the accuracy of each instrument with respect to standardized diagnostic tests and interviews for disorders of cognitive, motor, language and social-emotional development, as well as autism. Our specific aims are to: 1. Determine and compare the accuracy of three sets of developmental-behavioral screening instruments for use in pediatrics (i.e., sensitivity, specificity, PPV and NPV) in relation to results of standardized clinical evaluations for developmental and behavioral disorders. 2. Develop predictive models to integrate the results of developmental-behavioral screening tests with demographic data to improve detection of developmental and behavioral disorders. 3. Determine the impact of repeated administration of screening instruments on overall accuracy (i.e., cumulative sensitivity and specificity after repeated screening). Results of this study will markedly improve the evidence base to guide decisions about developmental- behavioral screening in very young children. If even a modest proportion of pediatricians adopted a more sensitive screening instrument, a large number of additional children with developmental-behavioral disorders would be identified and thus have the opportunity to receive appropriate interventions at a young age. A corresponding increase in specificity would result in a dramatic reduction in the number of false positive cases, thereby alleviating burden on primary care pediatricians, specialists, and parents.